Moderate CO2 Emissions Are Consistent with Historical Observations and Expert Judgements

Carbon emissions are the major driver of climate change and the resulting changes in risk. Projecting future anthropogenic carbon emissions is complicated by the presence of deep uncertainties, including fossil fuel resource constraints and the rate of future decarbonization. The Intergovernmental Panel on Climate Change addresses deep uncertainties through the use of discrete scenarios. However, risk assessments require probabilistic information. Here we use a simple integrated assessment model, calibrated using century-scale observations and expert assessments, to produce conditional probabilistic projections of carbon emissions through 2100. We project that, under a wide range of assumptions about these deeply uncertain factors, scenarios with approximate stabilization of future carbon emissions are consistent with historical observations and expert assessments. This finding agrees with recent projections of future emissions under current policies, though we identify a greater risk of more extreme emissions. We illustrate how the likely range of cumulative future emissions is shifted by changing assumptions about available fossil fuel resources and how the lower limit is increased by more pessimistic assumptions about decarbonization rates. Our results show that the most likely climate outcomes under current mitigation policies are more optimistic than under higher-end scenarios, though much more aggressive mitigation is required to reliably achieve the 2ºC Paris Accords target.